This invention relates broadly to radiant energy and is more particularly concerned with applications of ray generation and their transmission for purposes of detection, indication or measurement.
Radiant energy such as in the form of nuclear radiation has been increasingly used as a means to detect various physical parameters related, for example, to distance, measurement, object physical characteristics, etc. Various types of instruments have been designed using such radiant energy for purposes of long distance signalling, distance measurement and interlocking of equipment, and control of the operation of equipment. Such measurements have been termed, in some cases, transmission gauges wherein there is generally provided an aligned radiation source and radiation detector together with interconnected control apparatus which function together to detect, indicate or otherwise measure a physical characteristic of an object or material positioned or supported within a radiated beam established between the source and detector. An example of such a transmission gauge is shown in U.S. Pat. No. 3,373,286 wherein physical characteristics of the material to be tested are placed on a conveyor belt in a manner to pass within the radiated beam established between a radiation source and a detector.
Collimators have been employed with apparatus either in combination with a detector for purposes of what is termed focusing of the source as in U.S. Pat. No. 3,373,286 or in combination with the source to provide a defined radiation beam as produced from the radiation source, such as shown in U.S. Pat. Nos. 3,013,157 and 3,058,023. In the case where the collimator is part of the radiation source, the main purpose of the collimator is taught to be utilized to restrict the area or diametrical extent of the radiation beam. In the case of U.S. Pat. No. 3,013,157, the collimator strictly provides a means by which area or size of the radiation source itself can be measured, whereas in U.S. Pat. No. 3,058,023 the collimator provides the means for forming the gaseous molecules, as therein defined, which are evaporated from a liquid charged source and formed into a collimated beam.
However, it is not known to employ a source collimator in a radiant energy transmission system in a manner to maintain the radiation intensity so that it does not decrease as an inverse function of the square of the distance from the source collimator in applications necessitating long distance transmission of radiant energy where the radiated energy intensity level must be sufficiently higher than background radiation to be capable of detection by and, therefore, useful to the detector unit and interconnected control apparatus.
Prior art radiation transmission systems utilizing a relatively strong source shielded by a container having a single collimating hole have been employed in "unoccupied" (by humans) areas for long distance signaling. One usual manner of asssuring sufficient radiation intensity level in such systems is to provide a stronger radiation source. This increases the signal-to-noise ratio but is at the expense of surpassing the radiation intensity levels considered safe for working personnel. The dosage rate of 2.5 mr./hr. is the safe level standard set by the Atomic Energy Commission. A stronger source of itself is not the answer but rather the provision of a source which, when collimated, is effectively a low intensity source for safety purposes, yet provides a beam of sufficient strength to be easily detected at long transmission distances, such as 50 feet upwards to several hundred feet.
A particular application of concern relates to the detection of the presence or absence of an object moving on a conveyor system so as to operate at precise moments various types of equipment to automatically perform an operation on the object when in the presence of any such piece of equipment. In particular, the application involves an automated hot strip mill wherein detection means has been employed in the past to determine the location of a steel slab relative to a working station, such as a descaler or crop shear. The detection means previously employed was an infrared ray system having infrared energy source which has the disadvantage of not being completely reliable because steam or intense vapor from the descaling operation would interfere with the infrared signal causing the detecting system not to produce a signal indicative that a slab was or was not present at the descaler or near the entrance of the crop shear. What is needed, therefore, is a more reliable detection system wherein the steam or vapor from the descaling operation will not interfere with slab detection even though there may be a large distance involved between the energy source on one side of the strip mill lane and the detecting unit supported in aligned relation on the other side of the strip mill line.
In contemplating the employment of a detection system employing a gamma radiation signal which is not interfered with by steam or a heavy vaporous atmosphere, present practice would be to employ a radiation source that would exceed the safe exposure rate of 2.5 mr./hr. in order to produce a gamma radiation signal of sufficient intensity for long distance transmission, such as 50 to several hundred feet, as well as at a tolerable intensity above background radiation level. It is unusual to be able to obtain a sufficiently detectable gamma radiation signal above background radiation level at distances of several hundred feet without exceeding a safe exposure rate of gamma radiation.